1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to an apparatus which compensates for a variation in picture quality of an image forming apparatus, which forms an electrostatic latent image on a photosensitive member and develops the electrostatic latent image into a visible image, caused by a change in environment, aging or some other cause to allow an image of a stabilized gradation and tone to be formed.
2. Description of the Related Art
While an electrophotographic apparatus includes a large number of components, the charging characteristic of a photosensitive drum, the output intensity of a light source and the charging characteristics of a developer unit and a developer are factors which are unstable with regard to a variation in environment and passage of time, and accordingly, if the characteristics of the components vary, then the picture quality of an image of a half tone is liable to change. In a color printer, a variation in tone caused by a change in concentration of colors matters particularly with a half tone. However, a color adjusting operation is complicated very much, and automation is demanded in this regard. Simultaneously, stabilization in reproducibility in color is demanded for successive operations.
As a method for controlling the variation of the color reproducibility caused by a variation of the environment and passage of time, it is known to form a reference gradation pattern on a photosensitive drum, develop the pattern actually, detect the concentration of the thus developed image and feed back such detection signal to one or a plurality of ones of several image formation parameters.
The concentration detection method then may be 1) a method wherein the concentration is detected directly on the photosensitive drum, 2) another method wherein a reference pattern developed is transferred to the photosensitive drum and the concentration of the thus transferred image is detected or some other method. Whichever method is adopted, after detection of the concentration, the photosensitive drum or the transfer drum must be cleaned to remove the reference pattern.
Conventionally, while a reference concentration pattern is developed actually on the photosensitive drum and the gradation concentration of the developed image is detected by means of detection means as described above, difficulty is involved in processing of the developed image after detection. In particular, during successive operations, it is impossible to transfer the reference pattern for control to paper and discharge the paper, and accordingly, cleaning processing must be performed for the photosensitive drum to remove the reference pattern. However, since a pattern of a high concentration portion or a high concentration area applies a load to the cleaning unit, when the cleaning unit does not have a sufficient cleaning capacity, the pattern may possibly have a bad influence as a ghost upon a next print.
In the meantime, where a special adjusting mode is provided in which transfer to the transfer drum is performed, a reference concentration pattern transferring sheet or a cleaner for exclusive use must be prepared, and besides, an adjusting operation during successive operations is impossible.